The father who is a very tough man punished his son for being late, he locked his son in the room and put the key in a place that the kid couldn’t reach and hen try to beats him.
In the middle of the fight the father falls down due to his chest pain (Angina pectoris) The kid can’t reach the key to open the door.
They both locked inside. The father is near death when he feels guilt for his son so he reveals why he was treating him this way.